Widespread attention has been focused in recent years on the consequences of bacterial contamination contracted by food consumption or contact with common surfaces and objects. Allergic reactions to molds and yeasts are also a major concern. Respiratory infections due to viruses such as SARS (severe acute respiratory syndrome) coronavirus, and the H5N1 virus and mutations thereof, now commonly referred to as the avian flu or bird flu, have become major public health issues. In addition, significant fear has arisen in regard to the development antibiotic-resistant strains of bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE). In response to these concerns, manufacturers have begun incorporating antimicrobial agents into materials used to produce objects for commercial, institutional and residential use.
The antimicrobial properties of silver have been known for several thousand years. The general pharmacological properties of silver are summarized in “Heavy Metals” and “Antiseptics and Disinfectants: Fungicides; Ectoparasiticides”—by Stewart C. Harvey in The Pharmacological Basis of Therapeutics, Fifth Edition, by Louis S. Goodman and Alfred Gilman (editors), published by MacMillan Publishing Company, NY, 1975. It is now understood that the affinity of silver ion for biologically important moieties such as sulfhydryl, amino, imidazole, carboxyl and phosphate groups are primarily responsible for its antimicrobial activity. The attachment of silver ions to one of these reactive groups on a protein results in the precipitation and denaturation of the protein. The extent of the reaction is related to the concentration of silver ions. The diffusion of silver ion into mammalian tissues is self-regulated by its intrinsic preference for binding to proteins through the various biologically important moieties on the proteins, as well as precipitation by the chloride ions in the environment. Thus, the very affinity of silver ion to a large number of biologically important chemical moieties (an affinity which is responsible for its action as a germicidal/biocidal/viricidal/fungicidal/bacteriocidal agent) is also responsible for limiting its systemic action—silver is not easily absorbed by the body. This is a primary reason for the tremendous interest in the use of silver containing species as an antimicrobial i.e. an agent capable of destroying or inhibiting the growth of microorganisms, including bacteria, yeast, fungi and algae, as well as viruses.
In addition to the affinity of silver ions for biologically relevant species, which leads to the denaturation and precipitation of proteins, some silver compounds, those having low ionization or dissolution ability, also function effectively as antiseptics. Distilled water in contact with metallic silver becomes antibacterial even though the dissolved concentration of silver ions is less than 100 ppb. There are numerous mechanistic pathways by which this oligodynamic effect is manifested i.e. by which silver ion interferes with the basic metabolic activities of bacteria at the cellular level, thus leading to a bacteriocidal or bacteriostatic effect.
While it is well known that silver-based agents provide excellent antimicrobial properties, aesthetic problems due to discoloration is frequently a concern. This is believed to be due to several root causes, including the inherent thermal and photo-instability of silver ions, along with other mechanisms. A wide range of silver salts are known to be thermally and photolytically unstable, discoloring to form brown, gray or black products. Silver ion may be formally reduced to its metallic state, assuming various physical forms and shapes (particles and filaments), often appearing brown, gray or black in color. Reduced forms of silver in the form particles of sizes on the order of the wavelength of visible light may also appear to be pink, orange, yellow, or beige due to light scattering effects. Alternatively, silver ion may be formally oxidized to silver peroxide, a gray-black material. In addition, silver ion may simply complex with environmental agents (for example, polymer additives, catalyst residues, impurities, and surface coatings) to form colored species without undergoing a formal redox process. Silver ion may attach to various groups on proteins present in human skin, resulting in the potentially permanent dark stain condition known as argyria. While pure silver sulfate is colorless, it has been observed to decompose upon exposure to light to a violet color. While the formation of colored species of silver or of other additives that impart discoloration to a plastic composition or fiber may be undesirable in itself, given that colorants are often intentionally added to produce a specific desired coloration, it is of greater practical importance that any discoloration imparted by an additive be stable over the useful lifetime of the composition, fiber or object made therefrom.
In any given practical situation, a number of mechanisms or root causes may be at work in generating silver-based discoloration, complicating the task of providing a solution to the problem. For example, U.S. Pat. Nos. 6,468,521 and 6,726,791, disclose the development of a stabilized wound dressing having antibacterial, antiviral and/or antifungal activity characterized in that it comprises silver that is complexed with a specific amine and is associated with one or more hydrophilic polymers, such that it is stable during radiation sterilization and retains the activity without giving rise to darkening or discoloration of the dressing during storage. Registered as CONTREET®, the dressing product comprises a silver compound complexed specifically with either ethylamine or tri-hydroxymethyl-aminomethane. These specific silver compounds, when used in conjunction with the specific polymer binder carboxymethylcellulose or porcine collagen, are said to have improved resistance to discoloration when exposed to heat, light or radiation sterilization and contact with skin or tissue.
The point in time when discoloration of a composition associated with a silver-based additive appears can range from early in the manufacturing process to late in a finished article's useful life. For example, thermal instability can set in shortly after introduction of the silver-based additive into a high temperature melt-processed polymer, or much later during long-term storage of the material or finished article at lower (e.g. ambient) temperatures, sometimes referred to as long-term heat stability. Likewise, photo-instability can result from short-term exposure to high-energy radiation processing or radiation sterilization, or later from long-term exposure of the material or finished article to ambient light (for example, requiring ultraviolet (UV) stabilization). In addition, polymeric materials are well known to inherently discolor to some degree either during high temperature melt processing, or later due to aging in the presence of light, oxygen and heat. Thermoplastic polymers such as polyolefins are typically processed at temperatures between about 130-300° C. and will degrade under these conditions by an oxidative chain reaction process that is initiated by free-radical formation. Free radicals (R*) formed either along the polymer backbone or at terminal positions will react quickly with oxygen (O2) to form peroxy radicals (ROO*), which in turn can react with the polymer to form hydroperoxides (ROOH) and another free radical (R*). The hydroperoxide can then split into two new free radicals, (RO*) and (*OH), which will continue to propagate the reaction to other polymer chains. It is known in the art that antioxidants and light stabilizers can prevent or at least reduce the effects of these oxidative chain reactions. Antioxidant stabilizers are typically classified as (1) free-radical scavengers or primary antioxidants, and (2) hydroperoxide decomposers or secondary antioxidants. Hindered amine light stabilizers are believed to act in part as free-radical and peroxy radical scavengers through the formation of nitroxyl radicals.
Primary antioxidants are added to polymers mainly to improve long-term heat stability of the final fabricated article. Primary antioxidants are often called free radical scavengers because they are capable of reacting quickly with peroxy or other available free radicals to yield an inert or much less reactive free radical species, thus halting or slowing down the oxidative chain reaction process that leads to degradation. Primary antioxidants typically include, for instance, sterically hindered phenols, secondary aromatic amines, hydroquinones, p-phenylenediamines, quinolines, hydroxytriazines or ascorbic acid (vitamin C). Although aromatic amines are the strongest primary antioxidant, they are highly staining and seldom used in thermoplastics.
Secondary antioxidants are added to polymers mainly to provide needed short-term stability in melt flow and color during high temperature melt processing of the plastic material. They are believed to function by reacting with hydroperoxides to yield stable products that are less likely to fragment into radical species. Secondary antioxidants can usually be classified chemically as either a phosphorous-containing or a sulfur-containing compound. Phosphites such as triesters of phosphoric acid (P(OR′)3) are believed to react with hydroperoxides (ROOH) to form phosphates (OP(OR′)3) and alcohols (ROH). Elemental sulfur compounds and diaryl disulfides are reported to decompose hydroperoxides by generating sulfur dioxide. Thioethers (R1SR2) are believed to react with hydroperoxides (ROOH) to yield sulfoxides (R1SOR2) and alcohols (ROH). Sulfoxides may in turn destroy several equivalents of hydroperoxide through the intermediate formation of sulfenic acids and sulfur dioxide.
A third group of antioxidant stabilizers is commonly referred to as synergists. These materials may not be effective stabilizers when used alone, but when used in combination with another antioxidant a cooperative action results wherein the total effect is greater than the sum of the individual effects. Carbon black acts synergistically when combined with elemental sulfur, thiols or disulfides, whereas these materials are largely ineffective when used alone under the same conditions. Homosynergism is used to describe two stabilizers of unequal activity that work by the same mechanism. For example, two radical scavenging primary antioxidants might function synergistically if one were to transfer a hydrogen atom to the radical formed by the other, thus regenerating the latter stabilizer and extending its effectiveness. Alternatively, heterosynergism might result between a free radical scavenger and a nonradical hydroperoxide decomposer that act on different portions of the oxidative chain reaction process that leads to decomposition. Ultraviolet absorbers or metal deactivators in combination with radical scavengers have also been report to be heterosynergists. Some common thiosynergists include the esters of β-thiodipropionic acid, for example the lauryl, stearyl, myristyl or tridecyl esters. A widely used antioxidant package for polyolefins is the primary antioxidant 2,6-di-tert-butyl-4-methylphenol (BHT) with the thiosynergist dilauryl thiodipropionate (DLTDP).
A rapidly emerging application for silver based antimicrobial agents is inclusion in polymers used in plastics and synthetic fibers. A variety of methods is known in the art to render antimicrobial properties to a target fiber. The approach of embedding inorganic antimicrobial agents, such as zeolite, into low melting components of a conjugated fiber is described in U.S. Pat. Nos. 4,525,410 and 5,064,599. In another approach, the antimicrobial agent may be delivered during the process of making a synthetic fibers such as those described in U.S. Pat. Nos. 5,180,402, 5,880,044, and 5,888,526, or via a melt extrusion process as described in U.S. Pat. Nos. 6,479,144 and 6,585,843. Alternatively, deposition of antimicrobial metals or metal-containing compounds onto a resin film or target fiber has also been described in U.S. Pat. Nos. 6,274,519 and 6,436,420. Still, the formation of colored species of silver that impart discoloration to a plastic composition or fiber is clearly undesirable from both an aesthetic and a practical materials performance perspective.
In addition to the color instabilities inherent to silver and to polymeric materials themselves, silver ion imbedded in polymer composites may react with polymer decomposition products (for example, free radicals, peroxides, hydroperoxides, alcohols, hydrogen atoms and water), modifiers (for example, chlorinated flame retardants), stabilizers and residual addenda (for example, titanium tetrachloride, titanium trichloride, and trialkylaluminum compounds from Ziegler-Natta catalysts) to form potentially unstable colored byproducts. This greater complexity of potential chemical interactions further challenges the modern worker in designing an effective stabilizer package for polymers containing silver species.
A number of approaches have been taken in the past to improve the light stability of melt-processed polymer composites containing a silver-based antimicrobial agent. Niira et al. in U.S. Pat. No. 4,938,955 disclose melt-processed antimicrobial resin compositions comprising a silver containing zeolite and a single stabilizer (discoloration inhibiting agent) selected from the group consisting of a hindered amine (CHIMASSORB 944LD or TINUVIN 622LD), a benzotriazole, a hydrazine, or a hindered phenol. Reduction in long-term discoloration from exposure to 60 days of sunlight in the air is reported. Ohsumi et al. in U.S. Pat. No. 5,405,644 disclose a fiber treatment process in which the addition of a benzotriazole, such as methylbenzotriazole, to treatment solutions subsequently inhibits discoloration in fibers comprising a silver containing tetravalent-metal phosphate antimicrobial agent following one day exposure to outdoor sunlight. Herbst in U.S. Pat. No. 6,585,989 adds a silver containing zeolite to a chlorinated bisphenol ether antimicrobial agent (TRICLOSAN® 2,4,4′-trichloro-2′-hydroxydiphenyl ether) in polyethylene and polypropylene to yield improved UV stabilization (reduced yellowing) in accelerated weathering tests. Kimura in U.S. Pat. No. 7,041,723 discloses that for polyolefins containing an antimicrobial combination consisting of (a) a silver containing zeolite and either (b) a silver ion-containing phosphate or (c) a soluble silver ion-containing glass powder, some drawbacks of each antimicrobial agent are mitigated, including the reduction of discoloration from UV light exposure in accelerated weathering tests.
Copending and commonly assigned U.S. Ser. No. 11/669,830 (filed Jan. 31, 2007 by Blanton, Dontula, Jagannathan, Bishop, Sandford, and Barnes) describes thermoplastic compositions comprising polyolefins, primary antioxidants, secondary antioxidants, and silver salts. Herbst in WO 2007/042416 describes antimicrobial garments and footwear comprising a phenolic antibacterial agent, specific antifungal agents, and optionally a silver ion releasing agent, in polyester or polyvinylchloride resins. Schneider et al. in WO 2008/046746 describe durable acaricidal resin formulations comprising a dust mite killing agent (preferably thiabendazol) and, optionally, an antimicrobial agent consisting of a phenolic compound and/or silver metal or a silver complex or silver salt, incorporated into the bulk of a polymer.
We have discovered that polyolefin masterbatch composites comprising silver sulfate and large amounts (25 weight %) of polymeric hindered amine light stabilizers exhibit very poor light stability. Thus, there is a need to provide compositions and articles comprising thermoplastic polyolefins, silver sulfate and a hindered amine light stabilizer with improved light stability.